Adjustable steering column assembly for a motor vehicle

ABSTRACT

A steering column assembly for a motor vehicle is disclosed having lower and upper steering column members that are slidingly interengaged. The upper column member is attached to a structural part of the motor vehicle by a ride down mechanism that allows the upper steering column to move in the event of a collision. A load transfer means is provided to bypass the ride down mechanism when an adjustment lever is moved to a position allowing the position of the upper steering column member to be adjusted thereby preventing accidental damage occurring to the ride down mechanism due to operator abuse.

FIELD OF THE INVENTION

This invention relates to an adjustable steering column assembly for amotor vehicle and in particular to an adjustable steering columnassembly for a motor vehicle having an improved ride down mechanism topermit limited collapse of the steering column assembly during a crash.

BACKGROUND

It is known from, for example, GB-A-2,368,819 to provide a ride downmechanism for a steering column assembly comprising a mounting plate forfastening to part of a body structure of a motor vehicle and a skidbracket fastened to the mounting bracket by a pair of threaded fasteningmeans which are engaged with an elongate slot in the mounting plate. Theadvantage of such an arrangement is that the force required to producerelative sliding between the skid bracket and the mounting plate is setby the torque applied to the threaded fastening means and, by carefultightening of the fastening means, the force required to producecollapse of the ride down mechanism can be accurately set to arelatively low level so that in the event of an accident the steeringcolumn is readily collapsed.

It is however a problem with such an arrangement that abuse loads inputby an operator of the motor vehicle, particularly during adjustment ofthe position of the steering column assembly, may allow the fasteningmeans to loosen thereby reducing the clamping force with adverse effectson both collapse performance and perceived quality due to steeringcolumn rattle. It is also a potential problem with such an arrangementthat partial collapse of the ride down mechanism can be produced if veryhigh abuse loads are applied during adjustment of the steering columnand this could have an adverse effect in the event of a crash due to thereduced ride down travel remaining in the ride down mechanism.

It has therefore been proposed to use a ride down mechanism as shown inFIGS. 2 and 3. The ride down mechanism comprising of a mounting plate 20having four holes 21 therein to secure it to a cross car beam (notshown) and a carriage member 24 which is clamped to the mounting plate20 by a threaded fastener (not shown) that is engaged with alongitudinal slot 22 in the mounting plate 20 and an aperture 28 in askid plate 25 part of the carriage 24.

The carriage 24 also has two side plates 26a, 26b in each of which isformed a hole 27 used to support the shaft of an adjustment lever (notshown). Two capsules 29 a, 29 b are fastened to the skid plate 25 partof the carriage 24 for engagement with recesses 23 a, 23 b in a rearedge of the mounting plate 20.

The capsules 29 a, 29 b prevent the movement of the fastener in the slot22 and therefore prevent loosening of the fastener due to abuse loadsinput by an operator of the motor vehicle and also prevent loss of ridedown travel due to abuse loading.

In the event of a crash, the force applied to the steering column issufficient to shear the capsules 29 a, 29 b off the skid plate 25thereby allowing the skid plate 25 to slide along the mounting plate 20against the resistance to motion produced by the clamping forcetherebetween due to the threaded fastener.

It is however a disadvantage with such an arrangement that a relativelyhigh force has to be input into the capsules in order to shear them offbecause their fixing must be made sufficiently secure to resist anyabuse loads transferred during adjustment of the steering column. In theevent of a crash this is undesirable because it produces a skewedresistance in which there is a peak in resistance force at thecommencement of impact (due the additional force needed to shear off thecapsules) followed by a sudden drop to a lower more acceptableresistance after the capsules have been sheared off. This can result inan high occupant retardation rate during initial impact which is notbeneficial to the occupant impacting upon the steering column.

It is an object of this invention to provide a ride down mechanism for asteering column assembly that overcomes at least some of thedisadvantages associated with the prior art.

According to the invention there is provided an adjustable steeringcolumn assembly for a motor vehicle characterised in that the assemblycomprises a lower column member slidingly engaged at one end with anupper column member, a ride down mechanism connecting the upper columnmember to a fixed part of the motor vehicle so as to permit ride down ofthe upper column member when a force above a predetermined level isapplied to the upper column member, an adjustment mechanism having alocked state in which the upper column member is secured in one of anumber of positions and an unlocked state in which the upper columnmember can be moved by an operator within a predetermined range ofmovement wherein when the adjustment mechanism is in the unlocked statea load transfer means is engaged to bypass the ride down mechanism so asto prevent the transfer of axial load from the upper column memberthrough the ride down mechanism during adjustment of the upper columnmember.

The ride down mechanism may comprise a mounting bracket fixed to part ofthe body structure of the motor vehicle, a carriage connected to theupper column member, a clamping means to apply a predetermined clampingforce between the mounting bracket and the carriage so as to inhibitrelative sliding between the carriage and the mounting bracket duringnormal use but allowing relative movement between the carriage and themounting bracket when an axial force above a predetermined magnitude isapplied to the upper column member in the event of a vehicle crashwherein the clamping means is a fastener passing through an aperture inone of the carriage and the mounting bracket and a longitudinal slot inthe other of the carriage and the mounting bracket so as to provide aslideable connection therebetween.

Preferably, the fastener may be a threaded fastener that is tightened toprovide a desired clamping force.

The carriage may be connected to the upper column member by two rack andpinion sets the racks of which are fastened to the upper column memberand the pinions of which are mounted on the shaft the pinions beingengageable with the racks to secure the upper column member in one ofits adjustment positions.

The load transfer means may comprise a detent that is engaged with thelower column member when the adjustment mechanism is in the unlockedstate and which is retracted when the adjustment mechanism is in thelocked state.

The detent may engage with one of an aperture in the lower column memberand a recess in the lower column member.

The steering column assembly may be a telescopically adjustable steeringcolumn assembly and the aperture is a slot of sufficient length topermit the upper column member to move throughout a normal range oftravel but acting so as to transfer load directly from the upper columnto the lower column member when the upper column member reaches eitherend of its normal range of travel.

The steering column assembly may be a telescopically adjustable steeringcolumn assembly and the recess may be an elongate recess of sufficientlength to permit the upper column member to move throughout a normalrange of travel but acting so as to transfer load directly from theupper column to the lower column member when the upper column memberreaches either end of its normal range of travel.

Alternatively, the load transfer means may comprise a detent thatinteracts with the mounting bracket when the adjustment mechanism is inthe unlocked state so as to prevent relative movement between thecarriage and the mounting bracket and is retracted when the adjustmentmechanism is in the locked state so as to permit relative movementbetween the carriage and the mounting bracket when an axial force abovethe predetermined magnitude is applied to the upper column member in theevent of a vehicle crash.

The detent may interact with the mounting bracket by engaging with oneof a flange formed as part of the mounting bracket and an aperture inthe mounting bracket.

The adjustment mechanism may include an operator operable leverconnected to a shaft rotatably supported by the carriage and the detentmay be driveably connected to the shaft.

The adjustment mechanism may include an operator operable leverconnected to a shaft rotatably supported by the carriage and an abutmentsurface fastened to the shaft for rotation therewith and the loadtransfer means may comprise a detent in the form of an end stop wherein,when the operator operable lever is in a position corresponding to theunlocked state the end stop is co-operable with the abutment surface soas to transfer load directly from the upper column member to the endstop and, when the operator operable lever is in a positioncorresponding to the locked state, the end stop is non-engageable withthe abutment surface so as to permit normal ride down to occur.

The abutment surface may be on a flange formed as part of the operatoroperable lever.

The end stop may be fastened to a bulkhead support used to support alower end of the lower column member.

As yet a further alternative, the load transfer means may comprise aretention strap having one end connected to the mounting plate and atleast one detent formed at an opposite end that interacts with thecarriage when the adjustment mechanism is in the unlocked state so as toprevent relative movement between the carriage and the mounting bracket,the at least one detent being retracted when the adjustment mechanism isin the locked state so as to permit relative movement between thecarriage and the mounting bracket when an axial force above thepredetermined magnitude is applied to the upper column member in theevent of a vehicle crash.

Preferably, the strap may have two detents for engagement with thecarriage.

The strap may have two detents for interaction with the carriage and thetwo detents may interact with the carriage by one of spring biasing intocontact therewith or positive clamping to the carriage.

The or each detent may be arranged to fail when a force greater than apredetermined force is applied to it so as to facilitate normalfunctioning of the ride down mechanism in the event of a vehicle crashwith the adjustment mechanism in the unlocked state.

DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawing of which:

FIG. 1 is a schematic view of a motor vehicle having an adjustablesteering column and a ride down mechanism;

FIG. 2 is a plan view of a mounting plate forming part of a prior artride down mechanism;

FIG. 3 is a pictorial view of a prior art carriage for cooperation withthe mounting plate shown in FIG. 2;

FIG. 4 is a pictorial representation of a ride down mechanism accordingto a first embodiment of the invention;

FIG. 5 is a pictorial view of a retention strap forming part of the ridedown mechanism shown in FIG. 4;

FIG. 6 is an upside down end view of part of the ride down mechanismshown in FIG. 4 drawn so as to correspond with the orientation of theretention strap shown in FIG. 5;

FIG. 7 a is a side view of a steering column assembly having a ride downmechanism according to a second embodiment of the invention;

FIG. 7 b is a scrap view in the direction of arrow “P” on FIG. 7 a;

FIG. 8 is a side view of a steering column assembly having a ride downmechanism according to a third embodiment of the invention; and

FIG. 9 is a side view of a steering column assembly having a ride downmechanism according to a fourth embodiment of the invention.

DETAILED DESCRIPTION

With reference to FIG. 1 there is shown a motor vehicle 1 having anadjustable and collapsible steering column assembly 2. The steeringcolumn assembly 2 comprises of a tubular lower column member 10slidingly engaged with a tubular upper steering column member 11. Anupper drive shaft (not shown) is drivingly engaged with an upper end ofthe lower column member 10 by means of a spline drive therebetween. Anupper end of the upper drive shaft has a steering wheel 12 fastenedthereto and the upper drive shaft is rotatably supported by one or morebearings located in the upper column member 11. The lower end of thelower column member is connected to a bulkhead support 13 fastened to abulkhead 5 of the motor vehicle 1. The bulkhead support 13 transfersrotary motion from the lower column member 10 to a lower drive shaft(not shown) connected to a steering rack (not shown). The bulkheadsupport 13 also resists axial movement of the lower column member 10 ifan axial load “F” is applied along a longitudinal axis x-x of the lowercolumn member 10 via the steering wheel 12.

The upper column member 11 is attached to a cross-member 3 of the motorvehicle 1 by means of a ride down mechanism 4 (which can be of any ofthe embodiments described herein with respect to the invention). Theride down mechanism is operable to permit the upper column 11 to moverelative to the cross-member 3 when the load “F” applied to the steeringwheel 12 exceeds a predetermined load such as will be the case when themotor vehicle 1 is involved in a frontal collision and an occupantimpacts against the steering wheel or against an air bag fitted to thesteering wheel 12.

An adjustment mechanism of any known type is incorporated into the ridedown mechanism 4 so as to permit the position of the upper column member11 to be moved into one of a number of positions by an operator of themotor vehicle 1. The steering column assembly 2 may be adjustable forreach, for rake or for reach and rake and is secured in any of itspositions by operation of an operator operable lever 14.

Referring now to FIGS. 4 to 6, there is shown a first embodiment of aride down mechanism which forms part of an adjustable steering columnassembly according to the invention.

The ride down mechanism comprises of a mounting plate 120 having anumber of apertures 121 therein used to fasten it to part of the bodystructure of the motor vehicle 1 such as the cross member 3. Themounting plate 120 has a longitudinally extending slot 122 therein whichextends in the same direction as the intended direction of ride down.

A carriage 124 is clamped to the mounting plate 120 by means of athreaded fastener (not shown) which extends through an aperture (notshown) in a skid plate 125 part of the carriage 124. The skid plate 125is bounded on each side by a side plate 126 a, 126 b and is U-shaped incross-section. The carriage 124 is substantially the same as the priorart carriage 24 shown in FIG. 3 with the exception that no capsules areattached to it and the apertures 27 are elongate slots 117.

It will be appreciated that a sliding connection could also be obtainedvia a longitudinal slot in the carriage 124 and an aperture in themounting plate 120 or by an arrangement having longitudinal slots in themounting plate and the carriage 124.

An adjustment mechanism is incorporated with the ride down mechanism.The adjustment mechanism comprises of an elongate slot 117 in each ofthe side plates 126 a, 126 b in which is engaged a shaft 115 so that theshaft is rotatably supported by the carriage 124. An operator operablelever in the form of an adjustment lever 114 is connected to one end ofthe shaft 115 and the shaft ends in an enlarged head 116 at an oppositeend. A cam plate 118 is interposed between the adjustment 114 and one ofthe side plates 126 a such that rotation of the adjustment lever 114relative to the cam plate 118 in one direction draws the two side plates126 a, 126 b closer together to put the adjustment mechanism in a lockedstate and rotation of the adjustment lever 114 relative to the cam plate118 in an opposite direction from the locked position allows the twoside plates 126 a, 126 b to move apart to put the adjustment mechanismin an unlocked state.

Two clamp lugs 127 a, 127 b are fastened to a split bush 129 fastened tothe upper column member 11. The lugs 127 a, 127 b have apertures throughwhich the shaft 115 extends and are positioned between the two sideplates 126 a, 126 b. A cantilever spring 128 a, 128 b is interposedbetween each of the clamp lugs 127 a, 127 b and the respective sideplate 126 a, 126 b. The cantilever springs 128 a, 128 b have free endswhich bear against legs 131 a, 131 b of a retention strap 130.

The retention strap 130 has a central body portion 132 terminating atone end in a hook 133 and is connected at an opposite end to the twolegs 131 a, 131 b. It will be appreciated that the retention strap 130is made from a single piece of material and is bent to form the hook 133and the two legs 131 a, 131 b.

The hook 133 is used to connect the retention strap 130 to the mountingplate 120 but is will be appreciated that other means of connectioncould be used such as riveting or welding.

The two legs 131 a, 131 b are engageable with the side plates 126 a, 126b when the adjustment lever 114 is moved to the unlocked state therebypreventing relative motion between the carriage 124 and the mountingplate 120 in a column collapse direction by transferring any load inputinto the upper column member 11 directly into the mounting plate 120thereby bypassing the ride down mechanism. It will be appreciated thatrelative motion in an opposite direction is prevented by the position ofthe threaded fastener in the longitudinal slot 122.

That is to say, when the adjustment mechanism is locked the split bush129 clamps the lower column member 10 to the upper column member 11 toprevent relative movement therebetween during normal use, the clampingforce being set to provide a desired resistance to collapse and theretention strap 130 can move freely between the two cantilever springs128 a, 128 b because the effect of clamping has removed the load fromthese and so there is no additional resistance to motion between thecarriage 124 and the mounting plate 120. However, when the adjustmentmechanism is in the unlocked state the cantilever springs 128 a, 128 bbias the legs 131 a, 131 b into engagement with the side plates 126 a,126 b thereby preventing any movement of the carriage 124 relative tothe mounting plate 120.

The two legs 131 a, 131 b therefore function as engageable detents whichare arranged to fail when a load greater than a predetermined load isapplied to them so as to facilitate normal functioning of the ride downmechanism in the event of a vehicle crash with the adjustment mechanismin the unlocked state. In this case, the detents fail by being pushedout of engagement with the carriage 124 when a force above thepredetermined force is applied. The predetermined force is a forcesufficient to counteract or overcome the spring force applied by the twocantilever springs 128 a, 128 b. It will be appreciated that otherembodiments could be constructed in which the legs 131 a, 131 b could bedesigned to break when a force above the predetermined force is applied.The predetermined force will be a force greater than that expected dueto operator abuse.

It will be appreciated that any suitable adjustment mechanism could beused and that the invention is not limited to use with the adjustmentmechanism described above.

It will be further appreciated that instead of the cantilever springs128 a, 128 b biasing the legs 131 a, 131 b into engagement with the sideplates 126 a, 126 b a positive clamping mechanism operated by theadjustment lever 114 could be used so that when the adjustment lever isin a locked state the clamping mechanism is released and when thelocking lever 114 is moved to an un-locked state the legs 131 a and 131b are clamped to the side plates 126 a, 126 b.

With reference to FIGS. 7 a and 7 b there is shown a second embodimentof an adjustable steering column assembly having a ride down mechanismconstructed in accordance with the invention.

The steering column assembly comprises a tubular lower steering columnmember 210 which is attached at one end to a bulkhead support 213 and isslidingly engaged at an opposite end with an upper column member 211.The bulkhead support 213 transfers rotary motion from the lower columnmember 210 to a lower drive shaft (not shown) used to connect the lowercolumn member 210 to a steering rack (not shown). The bulkhead support213 also resists axial movement of the lower column member 210 if anaxial load is applied along a longitudinal axis of the lower columnmember 210 from a steering wheel (not shown).

An upper drive shaft 218 is drivingly engaged with an upper end of thelower column member 210 by means of a spline drive therebetween. Anupper end of the upper drive shaft 218 has, in use, a steering wheel(not shown) fastened thereto and the upper drive shaft 218 is rotatablysupported by a bearing located in the upper column member 211.

A ride down mechanism comprising of a mounting plate 220 fastened topart of the body structure of the motor vehicle 1 such as the crossmember 3 and a carriage 224 connected to the upper column member 211. Athreaded fastener 225 is used to clamp the carriage 224 to the mountingplate 220 so as to permit the carriage to slide relative to the mountingplate 220 when an axial load is applied to the upper column member 211in a crash so as to effect ride down of the upper column member 211.

An adjustment mechanism similar to that previously described withrespect to FIGS. 4 to 6 is incorporated with the ride down mechanism andincludes a shaft 231 rotatably supported by the carriage 224 and anadjustment lever 214 attached to the shaft.

A load transfer means in the form of a detent or cam 230 is fastened tothe shaft 231 for rotation therewith. The cam 230 is rotatable from afirst position when the locking lever 214 is in a position correspondingto a locked state of the adjustment mechanism in which it engages withan aperture or slot 235 in the lower column member 210 to a positioncorresponding to an un-locked state of the adjustment mechanism in whichit is not engaged with the slot 235. It will be appreciated that insteadof an aperture or slot the lower column member 210 could have a recessformed in it with which the cam 230 can engage.

That is to say, when the steering column assembly is a telescopicallyadjustable steering column assembly, the aperture is a slot ofsufficient length to permit the upper column member to move throughout anormal range of travel but acting so as to transfer load directly fromthe upper column to the lower column member when the upper column memberreaches either end of its normal range of travel or is an elongaterecess of sufficient length to permit the upper column member to movethroughout a normal range of travel but acting so as to transfer loaddirectly from the upper column to the lower column member when the uppercolumn member reaches either end of its normal range of travel.

Therefore, when the adjustment mechanism is in its locked state, theload transfer means in the form of the cam 230 has no effect on theoperation of the ride down mechanism and ride down can occur byovercoming the clamping force between the mounting plate 220 and thecarriage 224 and any friction between the upper and lower column members211 and 210 and any clamping force between the two column members 210,211 due to the adjustment mechanism but when the adjustment lever 214 ismoved to an unlocked position, the cam 230 is engaged with the slot 235in the lower column member 210 and any force applied to the upper columnmember 211 when the upper column 211 has reached its end of adjustmenttravel is then transferred via the carriage 224 and the shaft 231directly to the lower column member 210 thereby bypassing the ride downmechanism. It will be appreciated that any force transferred to thelower column member 210 from the cam 230 will be transferred through thelower column member 210 to the bulkhead support 213.

Therefore, controlled collapse of the ride down mechanism is assuredwithout the risk of operator abuse forces affecting the integrity of theride down mechanism.

With reference to FIG. 8, there is shown a third embodiment of anadjustable steering column assembly having a ride down mechanismconstructed in accordance with the invention.

The steering column assembly comprises a tubular lower steering columnmember 310 which is attached at one end to a bulkhead support 313 and isslidingly engaged at an opposite end with an upper column member 311.The bulkhead support 313 transfers rotary motion from the lower columnmember 310 to a lower drive shaft (not shown) used to connect the lowercolumn member 310 to a steering rack (not shown). The bulkhead support313 also resists axial movement of the lower column member 310 if anaxial load is applied along a longitudinal axis of the lower columnmember 310 from a steering wheel (not shown).

An upper drive shaft 318 is drivingly engaged with an upper end of thelower column member 310 by means of a spline drive therebetween. Anupper end of the upper drive shaft 318 has, in use, a steering wheel(not shown) fastened thereto and the upper drive shaft 318 is rotatablysupported by one or more bearings (not shown) located in the uppercolumn member 311.

A ride down mechanism comprising of a mounting plate 320 fastened topart of the body structure of the motor vehicle 1 such as the crossmember 3 and a carriage 324 connected to the upper column member 311. Athreaded fastener (not shown) is used to clamp the carriage 324 to themounting plate 320 so as to permit the carriage to slide 10 relative tothe mounting plate 320 when an axial load is applied to the upper columnmember 311 in a crash so as to effect ride down of the upper columnmember 311.

An adjustment mechanism for reach and rake is incorporated with the ridedown mechanism and comprises of is an operator operable adjustment levermovable between a locked position as indicated by the reference numeral314L and an unlocked position as indicated by the reference numeral314U, a rack and pinion locking mechanism 340 including two racks 341secured to the upper column member 211 and corresponding pinions (notshown) mounted on a shaft rotatably supported by the carriage 324. Theadjustment lever 314U, 314L is drivingly connected to the shaft and isheld in place by a nut.

Such a rack and pinion locking mechanism is described in U.S. Pat. No.4,541,298 but it will be appreciated that the invention is not limitedto the use of such a locking mechanism for the adjustment mechanism andthat any suitable adjustment mechanism could be used.

A load transfer means in the form of a strut 330 is connected to thebulkhead support 313, the strut 330 forms an end stop which isengageable with an abutment surface 317 (indicated as 317U when theadjustment mechanism is in an unlocked state and 317L when theadjustment mechanism is in a locked state) the end stop 330 cannotengage with the abutment surface 317 when the lever 314 is in the lockedposition/state 314L.

Therefore, if the adjustment mechanism is locked, the load transfermeans in the form of the strut 330 has no effect on the operation of theride down mechanism and ride down can occur by overcoming the clampingforce between the mounting plate 320 and the carriage 324 and anyfriction between the upper and lower column members 311 and 310 and anyclamping force between the two column members 310, 311 due to theadjustment mechanism. However, when the adjustment lever 314U is movedto an unlocked position, the strut 330 is engageable with the abutmentsurface 317U and any force applied to the upper column member 311 whenthe upper column 311 has reached its end of adjustment travel is thentransferred via the carriage 324 and the shaft directly to the bulkheadsupport 313 thereby bypassing the ride down mechanism.

Although the strut 330 is shown and described in this example as beingconnected to the bulkhead support 313 it will be appreciated that thestrut or another form of end stop could be fastened to some otherstructural part of the motor vehicle 1.

It will be appreciated that the strut 330 or the flange 317U could bearranged to fail when a force greater than a predetermined force isapplied to it so as to facilitate normal functioning of the ride downmechanism in the event of a vehicle crash with the adjustment mechanismin the unlocked state.

With reference to FIG. 9, there is shown a fourth embodiment of anadjustable steering column assembly having a ride down mechanismconstructed in accordance with the invention.

The steering column assembly comprises a tubular lower steering columnmember 410 which is attached at one end to a bulkhead support 413 and isslidingly engaged at an opposite end with an upper column member 411.The bulkhead support 413 transfers rotary motion from the lower columnmember 410 to a lower drive shaft (not shown) used to connect the lowercolumn member 410 to a steering rack (not shown). The bulkhead support413 also resists axial movement of the lower column member 410 if anaxial load is applied along a longitudinal axis of the lower columnmember 410 from a steering wheel (not shown).

An upper drive shaft 418 is drivingly engaged with an upper end of thelower column member 410 by means of a spline drive therebetween. Anupper end of the upper drive shaft 418 has, in use, a steering wheel(not shown) fastened thereto and the upper drive shaft 418 is rotatablysupported by one or more bearings (not shown) located in the uppercolumn member 411.

A ride down mechanism comprising of a mounting plate 420 fastened topart of the body structure of the motor vehicle 1 such as the crossmember 3 and a carriage 424 connected to the upper column member 411. Athreaded fastener (not shown) is used to clamp the carriage 424 to themounting plate 420 so as to permit the carriage to slide relative to themounting plate 420 when an axial load is applied to the upper columnmember 411 in a crash so as to effect ride down of the upper columnmember 411.

An adjustment mechanism for reach and rake is incorporated with the ridedown mechanism and comprises of an operator operable adjustment lever414 movable between a locked position as indicated in FIG. 9 and anunlocked position which is rotationally clockwise from the positionshown in FIG. 9 and a rack and pinion locking mechanism 440 includingtwo racks 441 secured to the upper column member 411 and correspondingpinions mounted on a shaft rotatably supported by the carriage 424. Theadjustment lever 414 is drivingly connected to the shaft and is held inplace by a nut. Such a rack and pinion locking mechanism is described inU.S. Pat. No. 4,541,298 but it will be appreciated that the invention isnot limited to the use of such a locking mechanism for the adjustmentmechanism and that any suitable adjustment mechanism could be used.

A load transfer means is provided in the form of a cam surface 430formed on the adjustment lever 414 and a detent in the form of a pin 431supported by a flange 434 on the carriage 424. The pin 431 has acircumferentially extending flange 432 formed on it which provides areaction surface for a coil spring 433 interposed between thecircumferentially extending flange 432 and the flange 434 on thecarriage 424. The spring 433 biases the pin against the cam surface 430at all times.

When the adjustment lever 414 is in a locked position as shown on FIG. 9the pin 431 is resting against the cam surface 430 and the ride downmechanism operates normally but when the adjustment lever 414 is rotatedin a clockwise direction into its unlocked position the effect of thecam surface is to drive the pin 431 upwards against the action of thespring 433 causing it to engage with an aperture 421 in the supportplate 420 thereby locking the carriage 424 to the support plate 420. Itwill be appreciated that instead of an aperture in the mounting plate420 the pin could abut against a flange attached to the mounting plate420.

Therefore, if the adjustment mechanism is locked the load transfer meansin the form of the strut, pin 430 has no effect on the operation of theride down mechanism and ride down can occur by overcoming the clampingforce between the mounting plate 420 and the carriage 424 and anyfriction between the upper and lower column members 411 and 410 and anyclamping force between the two column members 410, 411 due to theadjustment mechanism but when the adjustment lever 414 is moved to anunlocked position, the pin 430 is engaged with the aperture 421 in themounting plate 420 and any force applied to the upper column member 411when the upper column 411 has reached its end of adjustment travel isthen transferred via the carriage 424 directly to the mounting plate 420thereby bypassing the ride down mechanism formed by the slide connectionbetween the carriage 424 and the mounting plate 420.

It will be appreciated that the pin 430 could be arranged to fail when aforce greater than a predetermined force is applied to it so as tofacilitate normal functioning of the ride down mechanism in the event ofa vehicle crash with the adjustment mechanism in the unlocked state.

It will be appreciated by those skilled in the art that although theinvention has been described by way of example with reference to one ormore embodiments it is not limited to the disclosed embodiments and thatone or more modifications to the disclosed embodiments or alternativeembodiments could be constructed without departing from the scope of theinvention.

1. An adjustable steering column assembly comprising: a lower columnmember slidingly engaged at one end with an upper column member, a ridedown mechanism engaging said upper column member to permit ride down ofsaid upper column member when a force above a predetermined level isapplied to the upper column member; and an adjustment mechanism having alocked state in which said upper column member is secured and anunlocked state in which said upper column member can be moved whereinwhen said adjustment mechanism is in the unlocked state a load transfermeans is engaged to bypass said ride down mechanism to prevent thetransfer of axial load from said upper column member through said ridedown mechanism during adjustment of said upper steering column member.2. An assembly as claimed in claim 1 in which said ride down mechanismcomprises a mounting bracket fixed to part of a body structure of amotor vehicle, a carriage connected to the upper column member, aclamping means to apply a predetermined clamping force between themounting bracket and the carriage so as to inhibit relative slidingbetween the carriage and the mounting bracket during normal use butallowing relative movement between the carriage and the mounting bracketwhen an axial force above a predetermined magnitude is applied to theupper column member in the event of a vehicle crash wherein the clampingmeans is a fastener passing through an aperture in one of the carriageand the mounting bracket and a longitudinal slot in the other of thecarriage and the mounting bracket so as to provide a slideableconnection therebetween.
 3. An assembly as claimed in claim 2 whereinthe load transfer means comprises a detent that is engaged with thelower column member when the adjustment mechanism is in the unlockedstate and which is retracted when the adjustment mechanism is in thelocked state.
 4. An assembly as claimed in claim 2 wherein the loadtransfer means comprises a detent that interacts with the mountingbracket when the adjustment mechanism is in the unlocked state so as toprevent relative movement between the carriage and the mounting bracketand is retracted when the adjustment mechanism is in the locked state soas to permit relative movement between the carriage and the mountingbracket when an axial force above the predetermined magnitude is appliedto the upper column member in the event of a vehicle crash.
 5. Anassembly as claimed in claim 4 wherein the detent interacts with themounting bracket by engaging with one of a flange formed as part of themounting bracket and an aperture in the mounting bracket.
 6. An assemblyas claimed in claim 2 in which the adjustment mechanism includes anoperator operable lever connected to a shaft rotatably supported by thecarriage and an abutment surface fastened to the shaft for rotationtherewith and the load transfer means comprises of detent in the form ofan end stop wherein, when the operator operable lever is in a positioncorresponding to the unlocked state the end stop is co-operable with theabutment surface so as to transfer load directly from the upper columnmember to the end stop and, when the operator operable lever is in aposition corresponding to the locked state, the end stop isnon-engageable with the abutment surface so as to permit normal ridedown to occur.
 7. An assembly as claimed in claim 2 wherein the loadtransfer means comprises a retention strap having one end connected tothe mounting plate and at least one detent formed at an opposite endthat interacts with the carriage when the adjustment mechanism is in theunlocked state so as to prevent relative movement between the carriageand the mounting bracket, the at least one detent being retracted whenthe adjustment mechanism is in the locked state so as to permit relativemovement between the carriage and the mounting bracket when an axialforce above the predetermined magnitude is applied to the upper columnmember in the event of a vehicle crash.
 8. An assembly as claimed inclaim 7 wherein the strap has two detents for engagement with thecarriage and the two detents interact with the carriage by one of springbiasing into contact therewith or positive clamping to the carriage. 9.An assembly as claimed in claim 3 wherein each detent is arranged tofail when a force greater than a predetermined force is applied to it soas to facilitate normal functioning of the ride down mechanism in theevent of a vehicle crash with the adjustment mechanism in the unlockedstate.
 10. An assembly as claimed in claim wherein the adjustmentmechanism includes an operator operable lever connected to a shaftrotatably supported by the carriage and the detent is driveablyconnected to the shaft.